I still believe that I'm right and that Subversion shouldn't have an ever
increasing set of exceptions enshrined in svn_mime_type_is_binary() for all of
eternity. I can pursue the not-so-simple task of adding a new svn:text-type (a
boolean flag) that would override the normal MIME handling for purposes of
diff/merge only.

It would only be a small patch to add a whole bunch of exceptions to
svn_mime_type_is_binary(). Is that the wiser course? Am I just being a pedant
and a crank to insist that because some XML files should be treated as binary
that we can't make that sort of overarching assumption, just because it will
help most people?